A joist is a horizontal beam placed parallel in a set that carries the flat load above it. Joists can support a ceiling or a floor.
The exposed joists here span the room from wall to wall, or possibly the entire house.
These joists are held up by a drop beam. For flat ceilings covered in wallboard, a flush beam must be used.
Though many times salvaged beams are added as ceiling decoration, these joists are truly structural.
These joists are kept from twisting and warping by the wood blocks between them, above the two drop beams covered in wallboard.